Support for self-configuration and self-optimization of Long Term Evolution (LTE) mobile networks is defined in §22 of the Technical Specification (TS) entitled “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description”, published by the 3rd Generation Partnership Project (3GPP) in June 2009, ref. 3GPP TS 36.300 V9.0.0.
Self-configuration process is defined as the process whereby newly deployed evolved-NodeBs (eNB) are configured by automatic installation procedures to get the necessary basic configuration for system operation. This includes network connectivity, eNB authentication and registration to one or more gateway, downloading of eNB software and operational parameters, coverage/capacity related parameter configuration, neighbor list configuration. Self-Configuration process is further defined as the process whereby existing eNBs are automatically updated following the introduction of new eNBs.
Self-optimization process is defined as the process whereby User Equipment (UE) and/or eNB measurements are used to auto-tune the network. This includes neighbor list optimization, and coverage, capacity and handover control.
Automatic Neighbor Relation (ANR) function is an important feature of SON. The purpose of the ANR function is to relieve the operator from the burden of manually managing Neighbor Relations (NR). The ANR function resides in the eNB and manages the conceptual Neighbor Relation Table (NRT). The Neighbor Detection (ND) function finds new neighbors and adds corresponding NRs to the NRT, and the Neighbor Removal (NR) function removes outdated NRs.
A NR in the context of ANR is defined as an association from a source cell towards a target cell. For each cell that the eNB has, the eNB keeps a NRT. For each NR, the NRT contains the Target Cell Identifier (TCI), which identifies the target cell. For E-UTRAN, the TCI corresponds to the E-UTRAN Cell Global Identifier (ECGI) and Physical Cell Identifier (PCI) of the target cell. The NR comprises further attributes, such as whether the NR can be removed from the NRT, or whether there is a direct X2 connection with the eNB operating the target cell.
The ANR function works as follows.
The serving cell eNB has an ANR function. As part of the normal call procedure, the eNB instructs each UE to perform measurements on neighbor cells. The eNB may use different policies for instructing the UE how to perform measurements and when to report them to the serving eNB.
The UE sends a measurement report regarding a particular neighbor cell according to the configured measurement policy. This report contains the neighbor cell's PCI, but not its ECGI.
The eNB instructs the UE, using the newly discovered PCI as parameter, to read the ECGI, i.e. the PLMN Identity and the Cell Identity, and the Tracking Area Identifier (TAI), i.e. the PLMN Identity and the Tracking Area Code (TAC), of the neighbor cell.
When the UE has found out the new cell's ECGI and TAI, the UE reports the detected ECGI and TAI to the serving cell eNB. The eNB decides to add this NR in the NRT of the serving cell, and may use the reported ECGI to lookup a Transport Network Layer (TNL) address, e.g. an Internet Protocol (IP) address, to setup a new X2 connection with the eNB operating the detected neighbor cell.
If the serving eNB has no TNL address suitable for connectivity, then the eNB can utilize the configuration transfer functions to determine the TNL address as follows.
The eNB sends an eNB CONFIGURATION TRANSFER message to the Management Mobility Entity (MME) to request the TNL address of the candidate eNB, and includes relevant information such as the source and target eNB ID.
The MME relays the request by sending an MME CONFIGURATION TRANSFER message to the candidate eNB identified by the target eNB ID.
The candidate eNB responds by sending a further eNB CONFIGURATION TRANSFER message to the initiating eNB containing one or more TNL address to be used for X2 connectivity.
The MME relays the response by sending a further MME CONFIGURATION TRANSFER message to the initiating eNB.
In that RAN configuration data exchange, the MME is transparent, stateless, and do not keep track of the exchanged configuration data.
Release 10 of 3GPP has introduced Relay Nodes (RN) for extending the radio coverage to (mostly rural) areas where backhauling infrastructure are non-existent or deficient. E-UTRAN supports relaying by having an RN wirelessly connect to an eNB serving the RN, called Donor eNB (DeNB), via a modified version of the E-UTRA radio interface, the modified version being called the Un interface.
The RN supports the eNB functionality, meaning it terminates the radio protocols of the E-UTRA radio interface and the S1 and X2 interfaces.
In addition to the eNB functionality, the RN also supports a subset of the UE functionality so as to wirelessly connect to the DeNB.
The DeNB provides S1 and X2 proxy functionality between the RN and other network nodes (i.e., eNBs, MMES and S-GWs). S1 and X2 proxy functionality includes passing S1 and X2 data and control packets between S1 and X2 interfaces associated with the RN and S1 and X2 interfaces associated with other network nodes. Therefore, the DeNB appears to the RN as an MME or an S-GW for S1 interface, or as an eNB for X2 interface.
Handover towards a target cell operated by an RN is expected to work as follows.
In the HANDOVER REQUIRED message, the source eNB includes the TAI of the target cell and the eNB ID of a target DeNB in the Target ID field, and the ECGI of the target cell in the source to target container. The target MME is selected based on the TAI, and routes according to the target ID field. When the target DeNB receives the HANDOVER REQUEST message, it looks at the target ECGI and identifies the target cell as belonging to one of its connected RNs. The target DeNB can then further proxy the HANDOVER REQUEST message to the corresponding RN for further handling thereat.
This algorithm assumes the source eNB knows which DeNB controls the RN. Yet, whenever a UE reports to its serving eNB a newly detected neighbor cell operated by an RN, it will report its PCI, and its ECGI by means of the aforementioned ANR function. The ECGI includes the eNB ID of the RN (further referred to as the RN-ID). However the serving eNB needs the eNB ID of the DeNB (further referred to as the DeNB-ID) that controls that RN in order to build the HANDOVER REQUIRED message. This RN-ID/DeNB-ID mapping information is missing at the serving eNB.
As a consequence, handovers towards RN's cells work at the expense of heavy configuration and high Operational Expenditures (OPEX): whenever an RN is introduced in the network, the operator needs to configure all neighbor eNBs with the identity of the DeNB that controls that RN. This configuration is tedious, scalable with the number of deployed RNs which can be substantial, prone to human mistakes, and all the more tedious that RNs can be nomadic, i.e. can be moved from one place to another after being switched off and then switched on again in a new radio environment.